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Gopathy Purushothaman, David C Bradley; Single neuron sensitivity for a fine motion discrimination task. Journal of Vision 2003;3(9):198. doi: 10.1167/3.9.198.
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© ARVO (1962-2015); The Authors (2016-present)
AIM: The primate visual system is capable of discriminating minute differences (1–2 deg) in motion directions. Neural mechanisms underlying this acute psychophysical ability are unknown. The aim of this study was to determine whether single neurons can mediate such perceptual decisions.
METHODS: A rhesus monkey was trained to fixate a central dot while a reference RDK with upward-moving dots was presented. After a brief interval, a similar RDK moving in a direction 0.25–3 deg offset to one side of the reference was presented. The monkey indicated the side of the offset. Thresholds were estimated at 80% levels on the psychometric function. Responses of single MT neurons of various preferred directions were recorded for reference and test stimuli whose location, size and speed matched those of the RFs. Thresholds were estimated at 80% levels on the neurometric functions.
RESULTS: Most neurons (37/59) had a threshold 10–100 times the psychophysical threshold. The “best” neural threshold obtained (2/59) was 4 times the behavioral threshold. The mean threshold of 59 neurons was 35.7 deg and the mean psychophysical threshold (n=11) 1.8 deg.
CONCLUSIONS: Previous studies reported motion detection thresholds of single MT neurons to be similar to psychophysical thresholds for discriminating opposite directions [Newsome et al, 1989] indicating that broad pooling may result from anatomically coarse read-out mechanisms [Shadlen et al, 1996]. This study, using a fine discrimination task, showed motion discrimination thresholds of single MT neurons are significantly higher than psychophysical discrimination thresholds and established a functional necessity for broad pooling.
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